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APC BR1500G battery failure

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Aug 18, 2005
During one of our recent rains we lost power briefly and my UPS failed. It is one of two APC BR1500g UPS. It gave me a F04 error which is bad battery or a bad UPS. I pulled the battery from mine and replaced it with the battery from my wife's and the UPS functioned fine. I put the suspected bad battery in my wife's and the battery didn't. The battery was only 18 months old so I contacted APC and even though the warranty was only a year they are sending a replacement. They asked that I simply dispose of the bad battery pack in an appropriate manner. So I took it apart. The battery pack has 2 9ah 12 volt sealed lead acid batteries. I tested the voltage of both batteries and they were both 13.2volts. I tested the wires they were connected with in the pack with my multimeter and the wires and connectors were fine. I put everything back together and put it back in both UPS and both registered the battery pack as bad. I connected both batteries my smart charger and it says that they are full.

Any ideas on why this battery pack won't work? I tested the good battery pack and it registers 27.0-26.8 volt while the rejected one registers 26.6=26.4 volts. I wouldn't think that the slightly lower voltage would make a difference. Since both batteries have exactly the same voltage I wouldn't think that any of the cells in either are going bad.
Perhaps when under a load it fails? Similar to a car battery that shows good when not under a load but when cranked it fails. Just a guess.....
Test under load, like 10% of the amp-hour rating, or about 0.9 amp. A 12V incandescent bulb made for cars will work as a load, but don't use headlights because they draw several amps. Measure both the voltage with no load and the voltage after the 1 amp load has been applied for several seconds.